A data elementary buffer with underflow and overflow operational constraints is incorporated into a digital televison receiver to regulate the flow of data related to an ancillary data service to be presented in synchronization with a video or audio program element. The data elementary buffer ensures that data is received in time for decoding and presentation in synchronization with the video or audio element. The data elementary buffer also limits the amount of data that the receiver may be required to cache. The minimum size of the data elementary buffer is three times the size of a nominal data access unit or three times the quantity of data that the receiver can receive at the maximum rate in the period that a video element is displayed by the receiver.